Binding Problems with Maven-JAXB2-Plugin

What is irritating is that it was not possible to recreate the error without the plugin!
The code in wtf.java9.maven_jaxb2_plugin is a simplified (and accidentally “rightified”?) version of the code the plugin is running but it works on both Java 8 and Java 9 as indicated by another test.
Why?!

(Maybe the dependencies play a role?
Class path content between running the plugin and calling the API differs.)

Observe!

Running the project with mvn clean test creates two generated-jaxb-* folders, one for the sources created by the plugin the other by the JAXB API.
On Java 8 they are identical, on Java 9 they are not.
The tests try to verify that the sources are present and accordingly fail on Java 9.

API Use

In an effort to slowly approach the plugin’s implementation, particularly regarding the used EntityResolver different variants were implemented.
They can be seen in EntityResovlerFactory and selected with the Maven property entity.resolver, e.g.: